An MRI apparatus is an imaging apparatus configured to magnetically excite nuclear spin of an object placed in a static magnetic field with RF (Radio Frequency) pulses and generate an image by reconstructing MR (Magnetic Resonance) signals emitted from the object due to the excitation.
In the field of MRI, an imaging technique called DWI (Diffusion Weighted Imaging) is known. In DWI, a strong flow-encoding gradient magnetic field pulse called an MPG (Motion Probing Gradient) is applied. Application of an MPG pulse causes difference in MR signal intensity (i.e., contrast) between respective body tissues depending on difference in diffusion coefficient between those body tissues, and thus a diffusion-weighted image in which difference in diffusion coefficient is enhanced can be generated.
Degree of diffusion of each body tissue is indicated by an index called an ADC (Apparent Diffusion Coefficient). Further, as an index indicating intensity of an MPG pulse, a b-value (i.e., b-factor) is used.
In order to enhance contrast caused by difference in ADC, it is necessary to set a b-value to a sufficiently large value, and a value around 1000 is generally used for a b-value in many cases. However, when a malignant tumor is depicted, a much larger b-value is required in some cases because a b-value around 1000 is not large enough. On the other hand, it is not easy to set a b-value to a further larger value because of restriction in various elements such as limitation of hardware.
For the above reason, a technology has been developed, in which a diffusion-weighted image corresponding to a much larger b-value is obtained by computation from a diffusion-weighted image actually imaged with a b-value within a standard range. This technology is referred to as computed DWI.